(1) Field of the Invention
This invention relates to a substrate heat treatment apparatus for heat-treating substrates such as semiconductor wafers and glass substrates for liquid crystal displays (hereinafter simply called substrates). More particularly, the invention relates to a technique for heat-treating a substrate slightly spaced from a bake plate instead of being in contact with the plate.
(2) Description of the Related Art
A conventional apparatus of the type noted above has a bake plate variable in temperature, and three balls arranged on an upper surface of the bake plate to be located at apexes of an equilateral triangle in plan view (as disclosed in Japanese Unexamined Patent Publication No. 2000-193376, for example). With this apparatus, a substrate placed on the bake plate is supported at a lower surface thereof by the three balls, so that the entire lower surface is separated by a minute space from the bake plate instead of contacting the latter. By placing the substrate as separated by the minute space from the bake plate, the substrate is least affected by temperature variations occurring with the bake plate.
Heating treatment of substrates is an essential part of the semiconductor manufacturing process. Heating treatment carried out before development in the mask forming process using photoresist is also an important matter having a great influence on the accuracy of an etching mask size. With the processes becoming increasingly refined today, a high degree of temperature uniformity is required of the substrate heat treatment apparatus. Various improvements have been made to meet such requirements.
However, various films having different physical properties are combined into a multilayer structure in the actual semiconductor manufacturing process. With progress of the process, the substrate itself usually becomes curved (in the direction of thickness) due to differences in the rate of thermal contraction and expansion of the films. Thus, however accurately uniform a temperature distribution may be made over the surface of the bake plate, when a substrate is actually heated in the semiconductor manufacturing process, the minute space between the bake plate and substrate cannot be uniform over the entire surface of the substrate, but becomes uneven. This results in a problem that the heat treatment of the substrate is conducted without making full use of the highly precise temperature uniformity intrinsic to the bake plate.
In order to solve the above problem, a proposal has been made to divide temperature control of the bake plate into a plurality of areas, and to perform a different heating control for each area according to a distance between the substrate and bake plate. However, since the amount and direction of curvature of the substrate are not uniform, optimal temperature control must be carried out for each different substrate. Such control is difficult in actual situations. Even if it is actually practiced, the uniformity of heat treatment of substrates cannot be improved for all the complication of control.